The present invention relates to a data processing unit for executing instructions of different length. Modern processors are capable of processing instructions of different length. For example, a first or least significant byte of an instruction stream defines an operational code and therefore how many bytes of this particular instruction follow the first byte. These following bytes often define the respective operands of the specific operational code. In some embodiments of modern microprocessors the first byte does not define the length of an instruction but one of the following operands defines the final size of the instruction. Thus, an instruction can have any size within multiples of a byte. Nevertheless, the instruction fetch and decode units need to interpret the respective instruction to evaluate the size of the individual instructions to fetch the remaining parts of the respective instruction. This needs a lot of decoding logic and might slow down a respective processor in particular if an instruction is very long.
Therefore, some reduced instruction set computers only provide instructions with a single format. Hence, in particular 32 Bit or higher microprocessors need a lot of memory to store programs. Especially portable devices and small devices or low cost devices suffer under this requirement.